Remove an obsolete development task
Remove an obsolete development task

This commit removes an obsolete `:generate_cops_documentation` development task.
It's been a year since it was introduced in https://github.com/rubocop/rubocop/commit/1905f1b
and is no longer in use.

tejasbubane/rubocop,maxjacobson/rubocop,maxjacobson/rubocop,maxjacobson/rubocop,rrosenblum/rubocop,rrosenblum/rubocop,tejasbubane/rubocop,rrosenblum/rubocop,tejasbubane/rubocop